It may be summer, and the Olympics are in full swing, but we can’t really expect to see record-breaking propagation on the HF bands in the Northern hemisphere.
This often puzzles new (and not so new) hams. After all, HF propagation relies on sunlight and we have that in abundance in the summer.
Unfortunately, life is not that simple. While increased UV in the summer is guaranteed, as the angle the sun makes with the ionosphere in the northern hemisphere is increased, you actually get less overall ionisation of the all-important F2 layer.
This is due to a chemical change in the composition of the F2 layer in the summer compared with the winter.
There is an increase in the molecular-to-atomic composition of this layer in the summer, which makes it harder to ionise. As a result the F2 critical frequencies in June can be half what it they are in January.
This effect is called the Winter Anomaly.
At the same time D-layer absorption is very high in the summer, especially around local noon, which gives us a double whammy – lots of absorption and low F2 critical frequencies.
The net effect is that the bands much above 18MHz can remain stubbornly closed, apart from Sporadic E (Es) openings, which by August are less prevalent. And the lower bands are also less useful during the day due to absorption.
For this reason stay away from 80m (3.5MHz) in the daytime – 40m will be better around the UK as long as the critical frequency stays up near 7MHz. Twenty metres (14MHz) will probably remain the most reliable DX band both during the day and night, although don’t write off 18MHz (17m).
Better HF conditions will start to reappear in September – sunspots, solar flares and coronal mass ejections (CMEs) willing.
Don’t forget there is a free book about HF propagation available to download on the right.
Go to the August propagation charts from the UK
There is an old Chinese curse that says "May you live in interesting times". I thought it was a greeting, but apparently not!
Well, we are living in interesting times as far as HF propagation goes.
July is usually in the middle of the HF summer doldrums - not really good conditions to speak off, but with some Sporadic E (Es). But this month is being characterised by a mass of solar flares, coronal mass ejections (CMEs) and high solar flux index (SFI) levels.
So while high SFI numbers usually indicate good conditions on the upper bands, the flare actvity is causing havoc with the bands shut down and lots of noise. Couple this with summer thunderstorms (Medium Wave was just a wall of crashes and bangs on Thursday 28th June) and it is hard to know what to work.
But in amongst all this there are good openings to be had. On 4th July I worked K2K in New Hampshire on 15m CW. This was a station being run for the US Independence Day and the prop charts would suggest that no path would have been open on that band and at that time ((1415hrs GMT).
Others are chasing 1A0C - The Order of Malta’s Italian Relief Corps (CISOM)- which is in Rome, not Malta by the way. Its signals have ranged from barely audible to S9+ depending on the solar flares and CMEs.
The best advice is keep on eye on the solar data at
www.solarcycle24.com, and don't write the bands off too soon. And if you don't know what SFI, A and K indices are, take a look at the free HF Propagation book listed on the right.
73 and gud DX! Steve G0KYA
Go to the July propagation charts from the UK
or select another month
